The present invention relates to a gas discharge tube such as a deuterium lamp for spectrographic use in an qualitative or quantitative analysis.
The deuterium lamp has high output in ultraviolet region and provides a stable and continuous spectrum. Therefore, the lamp is widely available in spectrophotometers, fluorescent spectrometers and other optical devices which require such ultraviolet light sources in order to carry out ultraviolet spectrometry for measuring spectral transmission characteristics and spectral absorption coefficients etc. of material to be examined.
One example of a conventional gas discharge tube 1 (a deuterium lamp) is shown in FIGS. 1 through 3. The gas discharge tube 1 generally includes an anode 3, a cathode 8, a shield cover 4 for these electrodes and an outer envelope 12. The anode 3 is provided on an optical axis 2 defined within the outer envelope 12, and the anode 3 is surrounded by the shield cover 4. In front of the anode 3, a conical apertured portion 5 formed of a molybdenum is provided which is integrally assembled to the shield cover 4. As best shown in FIG. 3, the conical apertured portion 5 is provided with a small diameter bore portion 5a at the anode side and a conical surface portion 5b provided contiguously therewith. The small diameter bore portion 5a has an inner diameter of 0.4 to 2.0 mm and an axial length L2 of 0.5 mm. The conical surface portion 5b has an apex angle .theta. of 60.degree. and an axial length L1 of 1.3 mm.
In front of the conical apertured portion 5, a light transmitting hole 7 is open for allowing light to pass therethrough. Further, at one side of the conical apertured portion 5, the cathode 8 is provided. Three sides of the cathode 8 are surrounded by the shield cover 4. However, remaining one side of the cathode 8 is provided with a shield member 10' whose edge 14 defines an opening which is open with respect to an electron path 9 along which electrons directing toward the anode 3 are passed.
According to the deuterium lamp 1 of this type, deuterium gas having pressure of several Torrs is enclosed within the envelope 12 formed of the transparent glass such as fused silica or UV-transmitting glass. The envelope 12 provides a light emitting portion 13 which is positioned on the optical axis 2. The optical axis 2 extends in a line connecting between a center of the small diameter bore portion 5a and a center of the light transmitting hole 7 formed in the shield cover 4.
After preheat to the cathode 8, a trigger voltage is applied between the anode 3 and the cathode 8 for initiating arc discharge. After the discharge, a source voltage is applied for continuing the discharge. Thus, electrons pass along the flow line 9 and a plasma region 16 is provided on the conical apertured portion 5. The conical apertured portion 5 serves as an electron converging region. At the time of the arc discharge, sputtered materials are released from the cathode 8. Therefore, the shield member 10' prevents the sputtered material from the cathode 8 from being adhered onto the conical apertured portion 5 and a light emitting portion 13 of the glass envelope 1, to thereby obviate reduction in reflection efficiency and light transmittance. Incidentally, according to the above described arrangement, the cathode 8 is not positioned in confrontation with the anode 3, but is positioned offset therefrom. This is due to the fact that if the cathode 8 is positioned in directly front of the anode 3, light beam emitted from the conical apertured portion 5 is interrupted by the cathode 8. Further, the above described sputtered materials may be easily adhered onto the conical surface portion 5b and the light emitting portion 13. Furthermore, by the deviating arrangement of the cathode 8, the electron path length can be elongated by making use of the curved flow line, so that acceleration to the electron is obtainable at the conical apertured portion 5 in order to effectively provide the plasma arc thereat.